The present invention is related to power conversion, and in particular to a system and method for the efficient control of a power converter.
Converters are commonly employed to convert electrical energy from alternating current (AC) form to direct current (DC) form, or vice versa. In many applications, AC power is provided by a source and converted (i.e., rectified) to DC power for consumption by a load. To improve the power factor of the AC-to-DC conversion, active rectifiers are employed that include switches that are rapidly turned ON and OFF to ensure the current drawn by the active rectifier is sinusoidal and in-phase with the voltage provided by the AC source. Prior art active rectifier controllers use the AC output voltage of the generator as a reference and generate the control signals to maintain the AC current drawn by the rectifier in-phase with the AC output voltage (i.e., unity power factor). This type of control works well for applications in which the AC source, such as the power grid, is stable (i.e., the voltage is not significantly affected by the power drawn by an attached load). For example, high-frequency switching of the active rectifier does not significantly affect the AC output voltage provided by this stable AC source. Variable frequency synchronous generators (VFSG), such as those employed in aerospace applications, typically operate at or near their maximum rated power and frequency capability and exhibit an output impedance. Application of a load such as a high-frequency active rectifier to a VFSG exhibiting output impedance results in disturbances of the output voltage and consequently a phase shift between the output voltages of the VFSG and the back-electromotive force (BEMF) of the VFSG. Prior art methods that control a high-frequency active rectifier such that the current drawn by the rectifier is in-phase with the monitored output voltage results in the BEMF voltage of the VFSG being out of phase with the current provided by the VFSG. As a result, the overall efficiency of the VFSG (i.e., the power factor) becomes less than optimal.